As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Accordingly, much research on batteries corresponding to various needs thereof has been carried out.
Especially, the demand of a lithium secondary battery has sharply increased because the lithium secondary battery has a higher voltage than a conventional nickel-cadmium battery or a conventional nickel-metal hydride battery, and an increased number of charge and discharge cycles are possible for the lithium secondary battery.
In terms of its external shape, the demand of a rectangular battery and a pouch-shaped battery is high because the rectangular battery and the pouch-shaped battery have a small thickness, and therefore, the rectangular battery and the pouch-shaped battery can be easily applied to various products, such as a mobile phone. In terms of its material, the demand of a lithium secondary battery, such as a lithium cobalt polymer battery, is high because the lithium secondary battery has high energy density and discharge voltage.
FIG. 1 illustrates an exemplary structure of a pouch-shaped secondary battery 100.
Referring to FIG. 1, the secondary battery includes an electrode assembly 300, electrode taps 302 and 304 extending from the electrode assembly 300, electrode leads 400 and 410 welded to the electrode taps 302 and 304, respectively, and pouch-shaped case 200 for receiving the electrode assembly 300.
The electrode assembly 300 is a structural member including cathodes, anodes, and separators disposed between the cathodes and the anodes for isolating the cathodes and the anodes from each other, which are stacked successively in the order of one cathode, one separator, and one anode. The electrode taps 302 and 304 extend from the respective electrode plates of the electrode assembly 300. The electrode leads 400 and 410 are electrically connected to the electrode taps 302 and 304 extending from the respective electrode plates. The electrode leads 400 and 410 are partially exposed from the case 200. The case 200 provides a space for receiving the electrode assembly 300. In the case that the electrode assembly 300 is a stack-type electrode assembly as shown in FIG. 1, the inner upper end of the case 200 is spaced apart from the electrode assembly 300 such that the plurality of cathode taps 302 are attached to each other, which are then coupled to the electrode lead 400, and the plurality of anode taps 304 are attached to each other, which are then coupled to the electrode lead 410.
The electrode assembly 300 is placed in the case 200 while the electrode leads 400 and 410 are partially exposed to the outside, an electrolyte is poured into the case 200, and heat and pressure are applied to the edge of an upper case and the edge of a lower case, while the edge of the upper case and the edge of the lower case is in contact with each other, such that the edge of the upper case and the edge of the lower case can be securely fixed to each other by thermal welding. In this way, the pouch-shaped secondary battery 100 is completed.
Although the lithium secondary battery has many advantages as compared to the conventional nickel-cadmium battery or the conventional nickel-metal hydride battery, the lithium secondary battery has a problem in that the lithium secondary battery is weak. Specifically, the electrolyte is poured at the latter process during the manufacture of the battery. For this reason, an organic solvent having a low boiling point is frequently used. In this case, however, when the battery is overcharged or when the battery is left at high temperature, the electrode assembly or the battery case may swell due to the increase of the internal pressure of the battery. As a result, the case may be deformed. This deformation of the case may cause the explosion of the battery.
In order to solve the above-mentioned problem, a method of hardening a plane-type battery using ultraviolet rays or electron beams, and a method of coating gel to the electrode plates instead of pouring the electrolyte have been proposed (U.S. Pat. No. 5,972,539, U.S. Pat. No. 5,279,910, and U.S. Pat. No. 5,437,942). These conventional methods somewhat alleviate the swelling of the electrode assembly or the battery case. However, satisfactory stability is not guaranteed by these conventional methods.
Some conventional technologies propose a system in which a strain gauge type sensor is attached to the surface of the pouch-shaped battery, a protection circuit mounted between the terminals (the cathode and the anode) of the battery and the input and output terminals interrupts the operation of the battery based on a value detected by the pressure sensor. Specifically, when the case of the battery, i.e., the pouch, swells, the degree of the swelling is detected by the sensor, and the detected value is transmitted to the protection circuit, which interrupts the current flowing between the cathode and the anode when the detected value exceeds a predetermined level.
However, the above-described system for measuring the swelling of the battery case does not provide high reliability. Also, it is difficult to stably mount the sensor to the surface of the pouch-shaped battery for the purpose of accuracy measurement. For example, as the size and the weight of the battery have been reduced, it is very difficult to accurately measure the swelling of the battery case depending upon the change of the surface area of the battery case. Also, the strain gauge type sensor requires large area for accurate measurement. As a result, the heat dissipation from the battery is interrupted by the strain gauge type sensor, which increases temperature of the battery. Furthermore, the strain gauge type sensor cannot be used at all for a rectangular battery, the surface expansion of which is relatively small.
Meanwhile, the secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several small-sized cells for each device. On the other hand, medium- or large-sized devices, such as vehicles, use a battery module having a plurality of cells electrically connected with each other because high output and large capacity are necessary for the medium- or large-sized devices.
FIG. 2 illustrates an exemplary structure of a secondary battery module having a plurality of unit cells stacked one on another.
Referring to FIG. 2, the plurality of unit cells 101, 102, and 103 are stacked one on another with high integration. From the upper ends of the unit cells protrude electrode terminals 401, 411, 402, 412, 403, and 413. The unit cells 101, 102, and 103 may be arranged in different manners in the secondary battery module 500. Generally, the unit cells are stacked as shown in FIG. 2, which provides high integration. Also, the unit cells 101, 102, and 103 are spaced a predetermined distance from each other such that channels 600, in which heat generated during the charge and discharge of the unit cells is removed, are defined between the neighboring unit cells. Generally, a rectangular cell or a pouch-shaped cell is used as the unit cells 101 constituting the secondary battery module 500.
The safety problem of the secondary battery module may be more serious because the plurality of unit cells are stacked one on another in a small space. Specifically, the abnormal operation of some of the unit cells may reduce the abnormal operation of the other unit cells. Therefore, a measure for preventing the above-mentioned problem is urgently required. Until now, however, a technology for effectively guaranteeing the safety of the secondary battery module has not been proposed.